1. Field of the Invention
The present invention relates to a method and system for predicting acoustic fields, and more particularly to a method and system for accurately predicting acoustic fields on a prediction plane based on a source plane, a medium, and a measurement plane that make a relative movement.
2. Description of the Related Art
Many techniques for acquiring the hologram of acoustic fields using sound pressure measurements and deriving information about a sound source from the hologram have been proposed in both private and military sectors. Farfield directivity information, nearfield vector intensity information, surface velocity information, total sound power information, etc. can be obtained from the hologram of acoustic fields. A technology for detecting the location and strength of a sound source by using the hologram of acoustic fields may be used to find enemies in military applications and to detect and eliminate a noise source or build a noise wall in civilian industry sectors. Especially along with the recent increased attention towards the environment and living standards, there exists an increasing need for appropriately dealing with noise sources based on accurate information about the noise source.
Many acoustic holography techniques have been proposed. Some of the acoustic holography techniques are disclosed in J. D. Maynard, E. G. Williams, and Y. Lee “NearField Acoustic Holography (NAH): I. Theory of Generalized Holography and the Development of NAH”, Journal of the Acoustical Society of America, Vol. 74, No. 4, pp. 1395-1413 (1985), W. A. Veronesi and J. D. Maynard “NearField Acoustic Holography (NAH): II. Holographic Reconstruction Algorithms and Computer Implementation”, Journal of the Acoustical Society of America, Vol. 81, No. 5, pp. 1307-1322(1988), U.S. Pat. No. 4,415,996 entitled “Nonwavelength-Limited Holographic Acoustic field Reconstruction” by J. D. Maynard and E. G. Williams, J. Hald “Method of Spatial Transformation of Acoustic fields—A Unique Technique for Scan-Based Near-Field Acoustic Holography Without Restrictions on Coherence” Technical Review No. 1, 1989, BK publication, and Loyau, J. C. Pascal, and P. Galliard, “Broadband Acoustic Holography Reconstruction from Acoustic Intensity Measurement” Journal of the Acoustical Society of America, Vol. 84, No. 5, pp. 1744-1750 (1988).
Acoustic Holography (AH) is a technology for obtaining a hologram on a reference plane called a hologram plane and estimating the properties of sound waves at all spatial positions of interest by analyzing the hologram.
FIG. 1 conceptually illustrates a conventional AH technique. Referring to FIG. 1, the AH technique measures spatial acoustic fields, namely a hologram, using a microphone array having a plurality of microphones on an arbitary plane and estimates a spatial distribution of acoustic fields from the hologram. This AH technique is based on acoustic field mode interpretation that relies on a spatial Fourier transform based on the Kirchhoff-Hehmholts integral equation that is an acoustical interpretation theory of phase-coherent acoustic fields.
Now a description will be made of a Moving Frame Acoustic Holography (MFAH) technique proposed to improve the conventional AH technique. Before the MFAH technique was proposed, conventional measurement schemes required that the spatial position between the microphone array and the sound source should be fixed. Therefore, errors were inevitable when the sound source moved. Also, when air being an acoustic medium flows, the conventional measurement schemes have limitations in their application. Although studies were continuously conducted on the issue, no specific solution was found. In this context, the present inventors proposed an MFAH technique based on a single linear array for the first time, and a patent was granted for their MFAH technique (Korea Patent No. 217872). For details, see H.-S. Kwon and Y.-H. Kim, “Moving Frame Technique for Planar Acoustic Holography”, J. Acoust. Soc. Am., Vol. 103, No. 4, pp. 1734-1741(1998).
The MFAH technique estimates spatial information about a sound source using temporal data from sound pressure measurements according to the constant-velocity relative movement relationship between the sound source and microphones. With the use of known frequency and velocity information, a time-space variation is detected based on the idea that the Doppler shift reveals the time-space relationship. However, this MFAH technique derives a modulated wavenumber spectrum from a frequency spectrum. If modulated wavenumber spectra overlap because frequency components are close to each other, if a source plane or a prediction plane moves instead of a measurement plane, or if a medium moves, errors occur.
While the above MFAH technique is important in that it is the first to take into account the relative movement between a sound source and a hologram plane with regards to the conventional acoustic holography, it has many limitations in its effectiveness in real-world implementation, such as a normal-state acoustic field of a single frequency, the movement of measurement microphones, etc.
Especially when modulated wavenumber spectra overlap because frequency components are close to each other, when a source plane or a prediction plane moves instead of a measurement plane, or when a medium moves, severe problems are produced.